A geographic range limit is the geographic boundary beyond which a species does not occur, the limit or limits of the range of a species. "Core populations" are populations of individuals occurring within the center of the range, and "peripheral" or "edge populations" are groups of individuals near the boundary of the range.The inability of a species to expand its range beyond these limits is because of some factor or factors that constrain the species from adapting to overcome the factors that are imposing limits on its distribution.In some cases, geographical range limits are entirely predictable, such as the physical barrier of an ocean for a terrestrial species.In other cases, reasons why species do not pass these boundaries are unknown, however the main determinants of the distribution of a species across its range are ecological and evolutionary processes.For many species of invertebrate animals, the exact geographic range limits have never been precisely ascertained, because not enough scientific field work has been carried in many parts of the world to map distribution more precisely, therefore finding a range extension for species, especially marine species, is not an uncommon occurrence.